In a hydrostatic transmission, an abnormally high oil pressure may be caused in an oil path between a hydraulic pump and hydraulic motor during the time when the pump has been started up but the motor has not been driven to rotate yet. Such abnormally high oil pressure may be caused also during a normal operating condition of the hydrostatic transmission when load applied to the motor is suddenly increased. Further, an abnormally high oil pressure may be caused during the time when the pump has been stopped but the motor is still rotating due to its inertia. There is known from JP, U No. 3-100659 an oil pressure control valve assembly which is operable as a relief valve for excluding such abnormally high oil pressure and also as a relief valve for increasing oil pressure in the oil path between the pump and motor gradually after a start-up of the hydrostatic transmission so as to assure a shock-free start of a vehicle in which the hydrostatic transmission is employed.
This relief valve assembly is fashioned such that it relieves oil from the oil path between the hydraulic pump and hydraulic motor to an oil-charging path from a charging pump for supplementing working oil to the hydrostatic transmission. Relief valve member of the valve assembly is particularly composed of a differential piston which has a first pressure-applied surface area at the side of the oil path between the pump and motor and a second pressure-applied surface area which is located opposite to the first surface area and is larger than the first surface area. A throttled oil passage is associated with this differential piston for conducting oil pressure in the oil path between the pump and motor to the side of differential piston having the second oil pressure-applied surface area. A control piston is disposed co-axially with the differential piston, and a valve spring is provided such that it biases the differential piston through the control piston.
Consequently, an abnormally high oil pressure which may be caused in an oil path between the pump and motor during the time between a start-up of the pump and the start of rotation of the motor is excluded to the oil-charging path by an oil-relieving operation of the differential piston which is moved to an oil-relieving position by the oil pressure applied to the first surface area against the biasing force of the valve spring with pushing the control piston to move toward the spring. Then, oil pressure in the oil path between the pump and motor is gradually applied to the second pressure-applied surface area of differential piston, which is larger than the first surface area, through the throttled oil passage so that oil-relieving pressure of the differential piston is gradually increased. By this, oil pressure in the oil path between the pump and motor is gradually increased so that a shock-free start of the vehicle is assured. At a normal operating condition, the differential piston takes a position remotest from the control piston due to the difference in surface area between the first and second pressure-applied surface areas and, at such position, it blocks the oil path between the pump and motor from the oil-charging path. An abnormally high oil pressure, which may be caused during a normal operating condition of the hydrostatic transmission, as well as an abnormally high oil pressure which may be caused during an inertial rotation period of the motor after stopping the pump will also be excluded to the oil-charging path by the differential piston.
In a pressure control valve assembly of the known structure set forth above, the manner how the oil pressure in a fluid path between the hydraulic pump and motor is increased after the start-up of a hydrostatic transmission can be much influenced by the force of the valve spring and by the predetermined stroke of the differential and control pistons. The known valve assembly is not fashioned so as to permit adjustment of these factors. However, the manner of increase in oil pressure will provide much effect on the feeling of an operator at the time when a vehicle is started. Further, with respect to a working vehicle such as a tractor, it is desirable to adjust the manner of increase in oil pressure in response to a working or auxiliary implement, such as mower, trailer, blading device, cultivator or the like, to be used in conjunction with the vehicle or in response to the type of the vehicle such that, by way of example, when the inertial mass to be driven by the hydrostatic transmission is large, the oil pressure is increased as from a relatively high starting pressure at a relatively high rate.
Accordingly, a primary object of the present invention is to provide an improved pressure control valve assembly which permits a large variety of adjustment of the manner in which oil pressure in an oil path between a hydraulic pump and hydralic motor of a hydrostatic transmission is increased after the transmission is started up.